docs: def set(self, **kargs):
          """ Reset default keyword parameters.
  
          Assigns new default values from dictionary ``kargs`` to the fitter's
          keyword parameters. Keywords for the underlying :mod:`lsqfit` fitters
          can also be  included (or grouped together in dictionary
          ``fitterargs``).
  
          Returns tuple ``(kargs, oldkargs)`` where ``kargs`` is a dictionary
          containing all :class:`lsqfit.MultiFitter` keywords after they have
          been updated, and ``oldkargs`` contains the  original values for these
          keywords. Use ``fitter.set(**oldkargs)`` to restore the original
          values.
          """
          kwords = set([
              'mopt', 'fast', 'ratio', 'wavg_kargs', 'wavg_all',
              'fitterargs', 'fitname',
              ])
          kargs = dict(kargs)
          oldkargs = {}
          fargs = {}
          # changed
          for k in list(kargs.keys()):  # list() needed since changing kargs
              if k in kwords:
                  oldkargs[k] = getattr(self, k)
                  setattr(self, k, kargs[k])
                  kwords.remove(k)
              else:
                  fargs[k] = kargs[k]
                  del kargs[k]
          # unchanged
          for k in kwords:
              kargs[k] = getattr(self, k)
          # manage fitterargs
          if 'fitterargs' in kwords:
              # means wasn't in kargs initially
              oldkargs['fitterargs'] = self.fitterargs
              self.fitterargs = dict(self.fitterargs)
          if len(fargs) > 0:
              self.fitterargs.update(fargs)
          kargs['fitterargs'] = dict(self.fitterargs)
          return kargs, oldkargs